1. Field of the Invention
The present invention relates to a chrominance signal gain control circuit. More specifically, the present invention relates to an improvement in a manner of controlling the gain of an automatic color control circuit.
2. Description of the Prior Art
FIG. 1 shows a block diagram of a typical prior art automatic color control circuit commonly used in a color television receiver. An intermediate frequency signal is amplitude detected by a detector 9 and the resultant composite video signal is amplified by a video amplifier 10. The composite video signal, as amplified, is applied to a band pass transformer 11, where a chrominance signal as well as a burst signal are extracted. The chrominance signal as well as the burst signal, thus extracted, are applied to a first band pass amplifier 12 and further to a second band pass amplifier 13. A portion of the output from the first band pass amplifier 12 is applied to a burst amplifier 14, where the burst signal is extracted, as a function of a pulse output from pulse source circuit 15, and amplified. The pulse may be a flyback pulse obtainable from a horizontal scan circuit as the circuit 15 or of a delayed horizontal synchronizing pulse obtainable from a synchronization separating circuit as the circuit 15. The burst signal, as extracted and amplified, is applied to an automatic color control detecting circuit (ACC detector) 16 and is amplitude detected thereby. An output from the automatic color control detecting circuit 16 is applied to a control electrode of the first band pass amplifier 12 for control of the gain thereof as a function of the output from the circuit 16. It is to be understood that a closed loop for automatic color control is implemented by the first band pass amplifier 12, the burst amplifier 14, and the automatic color control detecting circuit 16, such that the burst signal output from the band pass amplifier 12 may be of controlled to be constant amplitude in spite of changes in the amplitude of the input signal. FIG. 2 is a graph showing relation between the gain of the first band pass amplifier 12 (ordinate) and the control voltage obtained from the automatic color control detecting circuit 16 (abscissa). As seen from the FIG. 2 graph, such a relation is rather linear.
More detailed description will be made of the operation of automatic color control with particular reference to FIG. 1. As seen from the FIG. 2 graph, the gain control characteristic of the first band pass amplifier 12 is of such a type that a control voltage increasing in a positive going direction as applied to the control electrode of the amplifier 12, increases the gain of the amplifier 12. Now assuming that the input signal to be applied to the first band pass amplifier 12 becomes larger, the output control voltage from the automatic color control detecting circuit 16 becomes lower and accordingly the gain of the amplifier 12 decreases, with the result that the magnitude of the output burst signal from the amplifier 12 is likely to be kept constant.
In general, the amplitude of the chrominance signal may range in magnitude up to a limit that satisfies the following equation: EQU V2 &lt; 2V1
where
V2 is the amplitude of the chrominance signal, and PA0 V1 is the amplitude of the burst signal in the band pass amplifier, as seen in FIG. 3, which shows a wave form of the chrominance signal as well as the burst signal.
However, the ratio of V1 versus V2 varies depending upon the broadcasting stations. Therefore, if the magnitude of the chrominance signal (V2) is controlled in response to the magnitude of the burst signal (V1), a disadvantage will result in practice. More specifically, proper automatic color control could be provided in reception from one broadcasting station, while the chrominance signal could be in a saturated condition in reception from another broadcasting station, although the magnitude of the burst signal has been controlled properly in both cases.